Tensioners as such are known in the art and are used to grip pipes or cables as they are deployed from a ship to the sea bed during offshore installation. The tensioners maintain a constant tension in the pipe or cable (hereinafter “member”) as it is deployed over the side of the vessel. Conventional tensioners consist of a number of traction devices having a track, each track comprising a number of contacting elements arranged on a continuous belt or loop. The track is driven so that as the contacting elements move with the belt or loop they move along a path or section in which contacting surfaces the contacting elements bear against the member and so control the passage of the pipe or cable through the tensioner. Typically two, three or four tracks are provided arranged equidistantly around the member. Providing a higher number of tracks (for example four rather than two) achieves a more even distribution of the loading on the member gripped by the tensioner and, for a given coefficient of friction between the tensioner and the member reduces the length over which the member has to be gripped in order to achieve a desirably low tensile force.
ISUs are relatively stiff but delicate “cables” which consist of a number of elements such as hoses, power cables and signal cables in a single package, generally of circular cross section. ISUs cannot withstand high clamping loads which are conventionally applied when laying pipes for example. In order to avoid such high clamping loads, ISUs require very long pipe tensioning machines (even where four tracks are provided) to provide the required grip on the ISU while providing sufficiently high hold-back tension (typically of the order of 60 tonnes) which is necessary for laying cable from a ship into very deep water.
Conventional traction devices are arranged so that the clamping forces on the member are reacted by the traction devices themselves, opposing traction devices being urged by hydraulic or mechanical means onto the member to provide the gripping force. The traction devices are thus required to resist both the tensile force resulting from the laying of the member and the gripping forces normal to the longitudinal axis of the member. This necessarily means that each traction device is relatively large and heavy and this problem is multiplied where higher numbers of traction devices are needed.